Add option to throw on non-empty RCON responses

Add option to FactorioServer.sendRcon to throw on non-empty responses
from Factorio.  This is useful for detecting errors returned by commands
that should not have a response.

lib/factorio/server.js

@@ -625,15 +625,22 @@ class FactorioServer extends events.EventEmitter {
 	 * wait until it is establied and then send the message.
 	 *
 	 * @param {string} message - message to send to server over RCON.
+	 * @param {boolean} expectEmpty -
+	 *     if true throw if the response is not empty.  Useful for detecting
+	 *     errors that might have been sent in response.
 	 * @returns {string} response from server.
 	 */
-	async sendRcon(message) {
+	async sendRcon(message, expectEmpty) {
 		this._check(["running"])
 		if (!this._rconReady) {
 			await events.once(this, 'rcon-ready');
 		}
 
-		return await this._rconClient.send(message);
+		let response = await this._rconClient.send(message);
+		if (expectEmpty && response !== '') {
+			throw new Error(`Expected empty response but got "${response}"`);
+		}
+		return response;
 	}
 
 	/**